1. Field of the Invention
This invention relates to solid-state photoelectrochemical cells having functions as both solar and secondary cells which can be widely utilized in various fields, including electronic watches or time pieces, permanent electric power sources for electronic computers, memory backup, photoelectric power generation and storage, and the like. The photoelectrical cell may often be referred to simply as "photocell".
2. Description of the Prior Art
A Si solar cell or battery is known as a typical physical photocell. A typical example of a chemical photocell known in the art is an optical water decomposition cell using TiO.sub.2. However, power source devices that function both as a solar cell and a secondary cell have not yet been practically realized. Existing cells are those in which a solar cell and a secondary cell are provided separately. In these cell systems, the solar and secondary cells are connected in parallel with a load. The power produced in the solar cell is stored in the secondary cell. When no light is expected as in the night or when an instantaneous large current is required, electric power is supplied to a load from the secondary cell.
In recent years, many attempts have been made using semiconductor electrodes in which an electrochemical reaction is caused to proceed by incidence of light on the electrodes to obtain useful substances and to store electric power. For instance, a cell is made using an n-TiO.sub.2 photo-electrode and a platinum electrode in which water is photochemically decomposed to obtain O.sub.2 gas at the photo-electrode and H.sub.2 gas at the platinum electrode. This cell is an example of obtaining useful gases or substances. However, the cell has not been reduced to practice yet as a photoelectrochemical cell although TiO.sub.2 has been evidenced as decomposing water by irradiation of light. This is because TiO.sub.2 has a large a band gap of about 3 eV with the following difficulties. An electromotive force cannot be generated in the absence of near ultraviolet or ultraviolet light, and the quantum efficiency is still not satisfactory. Also, a cell using water is more unlikely to be utilized than solid-state cells.
In Solid State Ionics, September 10, 1983, pp. 41-58, by North Holland Publishing Company, and Structure & Bonding, 49(1982), Springer Verlag, there has been shown the possibility of making an optical and secondary cell using an n-type ZrS.sub.2-y electrode which is capable of deintercalation of Cu.sup.+ ions. However, when the n-type ZrS.sub.2-y photo-electrode is used along with an aqueous electrolyte solution, a photo-corrosion reaction with the aqueous electrolyte solution as well as the photo-charging reaction undesirably takes place, with an attendant disadvantage that the life of the cell becomes very short.